Use system web browser under WSL

Launch `cmd.exe /c "start URL"` under WSL for both `fish_config` and
`help`. This works around #4299 but does not address the underlying
issue (#1132).

share/functions/help.fish

@@ -119,6 +119,11 @@ function help --description 'Show help for the fish shell'
             set fish_help_page "index.html"
     end
 
+    set -l wsl 0
+    if uname -a | string match -qr Microsoft
+        set wsl 1
+    end
+
     set -l page_url
     if test -f $__fish_help_dir/index.html
         # Help is installed, use it
@@ -150,8 +155,10 @@ function help --description 'Show help for the fish shell'
         end
     end
 
+    if test $wsl -eq 1
+        cmd.exe /c "start $page_url"
     # If browser is known to be graphical, put into background
-    if contains -- $fish_browser[1] $graphical_browsers
+    else if contains -- $fish_browser[1] $graphical_browsers
         switch $fish_browser[1]
             case 'htmlview' 'x-www-browser'
                 printf (_ 'help: Help is being displayed in your default browser.\n')

share/tools/web_config/webconfig.py

@@ -35,6 +35,14 @@ def isMacOS10_12_5_OrLater():
     version = platform.mac_ver()[0]
     return version and LooseVersion(version) >= LooseVersion('10.12.5')
 
+def is_wsl():
+    """ Return whether we are running under the Windows Subsystem for Linux """
+    if 'linux' in platform.system().lower():
+        with open('/proc/version', 'r') as f:
+            if 'Microsoft' in f.read():
+                return True
+    return False
+
 
 # Disable CLI web browsers
 term = os.environ.pop('TERM', None)
@@ -1139,6 +1147,8 @@ fileurl = 'file://' + filename
 print("Web config started at '%s'. Hit enter to stop." % fileurl)
 if isMacOS10_12_5_OrLater():
     subprocess.check_call(['open', fileurl])
+elif is_wsl():
+    subprocess.call(['cmd.exe', '/c', "start %s" % url])
 else:
     webbrowser.open(fileurl)